iTunes supports several types of video files, the most common being MP4 and MOV file extensions. QuickTime Player, which works in conjunction with iTunes, converts video to video or video to audio. You can export/convert video files to audio files like WAVs or AIFFs in QuickTime, which then can be converted in iTunes to MP3s. Since MP3s are audio files, there will be no video in your output.
Download QuickTime Player. Install and open. Click "File" and "Open File." Choose your iTunes video in the search finder and click "Open."
Once your file appears in the player, go to "File" and "Export." Click on the "Export:" tab near the bottom of the window. Scroll down the options and select "Sound to AIFF." Choose your file's output destination and click "Save."
Find your new .aiff file and open it in iTunes. On the toolbar at the top of the screen, select "iTunes" then "Preferences." Click on the "Import Settings" bar. Click on the "Import Using" toolbar and select "MP3 Encoder." Click "OK" twice to exit.
Right-click on your AIFF file in iTunes. Select "Create MP3 Version."